According to guidelines from the American Congress of Obstetrics and Gynecologists, women who exercise during pregnancy are advised to “avoid high heat and humidity to protect against heat stress.” The reason is to avoid any conditions that would raise a pregnant woman’s core temperature above 102.2 degrees Fahrenheit—a temperature that has been shown to increase the risk of fetal malformations and pregnancy complications.

However, without clear definitions of “heat” or “humidity,” these guidelines aren’t super helpful for moms-to-be who are wondering whether they can still go out for a run during summertime.

To find numbers to back up these terms, an international team of researchers conducted a systematic review of studies that have been done on pregnant women, exercise, and core temperature. Using evidence from 12 experimental design studies, the researchers concluded that, regardless of gestational period, pregnant women can safely exercise for up to 35 minutes at 80 to 90 percent max heart rate in 77 degrees and 45 percent relative humidity.

In essence, this means pregnant women are safe to go for a pretty hard run—i.e., the kind where you're breathing too hard to talk in full sentences—on an average summer day. (And, according to the researchers’ findings, they can go for an even longer swim, and relax afterward in hot tubs or saunas for up to 20 minutes.)

However, because every woman and every pregnancy is different, it’s dangerous to make any sweeping generalizations or assume that any “max threshold” will apply to every single woman. For instance, during her pregnancy, professional triathlete and gold medalist Gwen Jorgensen ran on 100-degree days in Portland, Oregon (while keeping water close at hand, of course), and while many people told her that she’d feel most comfortable while swimming, she told Runner's World, “It didn't feel right, so I cut back in the water and increased my running because that’s where I felt best.”

On the other end of the spectrum, “There are some people who should not exercise during pregnancy,” Lia Wrenn, M.D., a gynecologist and obstetrician at Affiliates in Ob/Gyn in Burlington, Vermont, told Runner's World. “Higher-risk people: anyone with significant lung disease, or any heart disease that’s valve-related and would increase the risk of heart attack. Anyone at risk of pre-term labor.”

Furthermore, while 77 degrees and 45 percent humidity might sound balmy to someone living in the deep south, any runner who has trained through a northeastern winter and lined up at the Boston Marathon will tell you that these same conditions can feel blistering if you’re not acclimated—pregnant or not.

“Women in Denver probably don’t have the same tolerance to humidity or heat as people in Florida,” said Abigail Bales, DPT, C.S.C.S., and owner of Reform Physical Therapy, a Manhattan-based practice that specializes in pregnant and postpartum active females, “but they have acclimated to altitude.” So what is right for someone living in one place won’t necessarily be right for someone living somewhere else.

Plus, even if a woman lives in the same place with the same weather, this doesn’t mean she will be able to treat every pregnancy the same way. “I ran through my first pregnancy to 32 weeks,” Bales said. She had been marathon training before she got pregnant, so running throughout August, September, and October was not a problem because her body was both fit and acclimated, and on days when the heat got to be too much, she took her running indoors.

With her second pregnancy, however, she had symphysis pubic pain early on, so, being a pelvic floor specialist, she stopped running entirely and relied strictly on weight training and spinning to stay fit—and sane.

“Exercise is great for stress release, and psychological wellbeing, too,” Wrenn said. She highly recommends exercise to her patients, pointing to not only psychological health, but also other benefits such as reduced risk of gestational diabetes, decreased likelihood of a C-section, and enhanced postpartum recovery time.

The important thing is to be smart and not overdo it. “We don’t recommend any contact sports, and anything with a high risk of falling,” Wrenn said. “But swimming, running, jogging, walking, strength-training—all great.”

If you experience any of the following symptoms, stop exercising and call your doctor:

Chest pain

Dizziness

Regular painful contractions

Vaginal bleeding

Amniotic fluid leakage

Allison GoldsteinContributing WriterAllison Goldstein is a freelance writer and editor who is endlessly fascinated by the scientific “why” of things.

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